1. Field of Invention
The invention relates to a method and an apparatus for locating defective fuel rods within fuel assemblies of water cooled nuclear reactors, and the like.
2. Description of the Prior Art
The core of a light water-cooled reactor, for instance, typically consists of about 40 to 50 thousand fuel rods which are usually arranged in groups of about 200 rods to form a fuel assembly. A fuel assembly for a reactor of this nature consists of two end fittings, control rod guide tubes and spacer grids for positioning the rods. The fuel rods consist of Zircaloy-4 cladding tubes which contain the fuel in oxide form and are closed at both ends with welded caps.
During prolonged operation, several rods may develop leaks so that cooling water can seep in or radioactive material can escape. The coolant purification system of the nuclear reactor is capable of handling a certain amount of radioactive fission products. However, it is desirable to keep the radiation level as low as possible in order to protect the operating personnel. Accordingly, the fuel assemblies are usually subjected to a so-called "seepage test" during shutdowns of the reactor, e.g. during refueling. The fuel assembly is placed in a water filled storage tank for this test. The fuel rods and the water heat up by residual decay. If a fuel assembly contains a defective rod, the fission products escape during the heating into the water. Through sampling of the water it can be determined whether the fuel assembly contains defective rods. This method is a totalizing method which determines only whether the fuel assembly contains defective rods. It cannot, however, identify the position or the location of the defect.
U.S. Pat. No. 3,983,741 suggests removing the upper end fitting from the fuel assembly, and slipping an immersion cask over the exposed fuel rod end caps while keeping the upper ends of the fuel rods above the water. Water seeping into defective rods evaporates due to the decay heat. Instrumentation can detect temperature differences of rods containing steam.
It is known from U.S. Pat. No. 3,945,245 to remove the end fittings from the fuel assembly, to slip a heating element on the endcaps of the fuel rods and to detect the generation of steam or condensate in the rods containing water by ultrasonic means.
Therefore, in accordance with these patents, it is possible to locate defective fuel rods. A disadvantage of these methods is that the end fitting of the fuel assembly must be removed and special provisions must be made to evaporate the ingressed water before the defective rods can be found by the instrumentation.
A further disadvantage is that in many fuel assembly designs only the lower end fitting is removable. The lower fuel assembly end fitting is installed in the reactor at the bottom end of the fuel rods. Therefore, before this end fitting can be removed the fuel assembly must be turned 180 degrees under the water in the fuel storage pool, an additional time consuming operation.
The seepage test, the disassembly of the end fitting and the evaporation of the leaked-in water, require a time consumption which is a loss in availability of the power plant. In addition, every operator of a nuclear power plant strives to keep the testing times of the fuel elements at a minimum to reduce the exposure time of the maintenance crew. When the allowable exposure limit is exceeded, a new crew must be employed.
Thus, a need has arisen to find a simpler inspection method for locating defective fuel rods which could reduce the required time and minimize the radiation exposure of the maintenance crew.